**Core Concept**
Hand and foot syndrome, also known as palmar-plantar erythrodysesthesia (PPE), is a common side effect of certain chemotherapeutic agents. It is characterized by erythema, swelling, and peeling of the skin on the palms and soles. The pathophysiology of hand and foot syndrome involves the accumulation of chemotherapy drugs in the skin, leading to damage to the rapidly dividing cells.

**Why the Correct Answer is Right**
5-Fluorouracil (5-FU) is a pyrimidine analog used in the treatment of various cancers, including colorectal, breast, and skin cancers. The mechanism of hand and foot syndrome in 5-FU involves the inhibition of thymidylate synthase, which leads to the accumulation of toxic metabolites in the skin. The rapid proliferation of skin cells in the palms and soles makes them more susceptible to the cytotoxic effects of 5-FU, resulting in the characteristic symptoms of hand and foot syndrome. The use of leucovorin, a thymidylate synthase inhibitor, can increase the risk of hand and foot syndrome by further inhibiting the enzyme.

**Why Each Wrong Option is Incorrect**
**Option A:** Vincristine is a vinca alkaloid that primarily causes neurotoxicity, including peripheral neuropathy, rather than hand and foot syndrome.
**Option B:** Cisplatin is a platinum-based chemotherapeutic agent that causes nephrotoxicity, ototoxicity, and neurotoxicity, but it is not commonly associated with hand and foot syndrome.
**Option D:** Azathioprine is an immunosuppressive agent used in the treatment of autoimmune disorders and transplantation, and it is not typically associated with hand and foot syndrome.

**Clinical Pearl / High-Yield Fact**
When using 5-FU, it is essential to monitor patients for signs of hand and foot syndrome, particularly in those with a history of prior radiation therapy or chemotherapy. Early recognition and management of hand and foot syndrome can help prevent severe complications and improve patient outcomes.

**✓ Correct Answer:** C. 5 FU